Ceramic materials are of critical importance for a number of high temperature, high performance applications such as gas turbines. These applications require a unique combination of properties such as high specific strength, high temperature mechanical property retention, low thermal and electrical conductivity, hardness and wear resistance, and chemical inertness. Design reliability and the need for economical fabrication of complex shapes, however, have prevented ceramic materials from fulfilling their potential in these critical high temperature, high performance applications.
The design reliability problems with ceramics, and the resultant failure under stress, are due largely to the relatively brittle nature of ceramics. This, in combination with the high cost of fabricating complex shapes, has limited the usage of ceramics.
Ceramics made from organosilicon polymers have the potential to overcome these problems. To this end, polymers based on silicon, carbon and/or nitrogen and oxygen have been developed. See, for example, "Siloxanes, Silanes and Silazanes in the Preparation of Ceramics and Glasses" by Wills et al, and "Special Heat-Resisting Materials from Organometallic Polymers" by Yajima, in Ceramic Bulletin, Vol. 62, No. 8, pp. 893-915 (1983), and the references cited therein.
The major and most critical application for ceramics based on polymer processing is high strength, high modulus, reinforcing fibers. Such fibers are spun from organosilicon preceramic polymers and are subsequently converted to ceramic materials, in particular, silicon carbide/silicon nitride bearing fibers by a two-step process of curing to render the preceramic polymeric fibers insoluble followed by pyrolyzation comprising heating the fiber in an inert atmosphere up to about 2,000.degree. C. whereupon the fibers are converted to ceramic form.
U.S. Pat. No. 3,853,567 is an early example of thermally treating a polysilazane resin to form ceramic articles comprising silicon carbide and/or silicon nitride. Thus, in Example 1 of the patent, a carbosilazane resin is formed, spun into filaments, the filaments rendered infusible by treating them with moist air for 20 hours at 110.degree. C. and subsequently heated over the course of 7 hours to 1,200.degree. C. in a nitrogen atmosphere and then to 1,500.degree. C. over the course of 5 minutes. A black-glistening filament which is completely insensitive to oxidation at 1,200.degree. C. and is amorphous to x-rays is disclosed as obtained. Subsequent heating to 1,800.degree. under argon produced a fiber consisting of .beta.-SiC, a little .alpha.-SiC and .beta.-SiC.sub.3 N.sub.4. Pyrolysis can be undertaken in inert gases such as nitrogen mentioned above as well as ammonia, argon or hydrogen.
Although the mechanical properties of silicon carbide fibers are not quite as good as those of high strength carbon fibers, SiC fibers will likely find increasing application in electronic components including epoxy composites therefor because such fibers are between five and six orders of magnitude less conducting than carbon fibers. This higher resistivity can be translated directly into a lower reflectivity for various forms of electromagnetic noise.
Unfortunately, the conductivity and dielectric constant of silicon carbide fibers formed from the pyrolysis of organosilicon preceramic polymers are much higher than those of crystalline silicon carbide. One possible source of this is the excess carbon in silicon carbide fibers formed by the pyrolysis of organosilicon preceramic polymers. It appears that the conduction mechanism in silicon carbide fibers is variable range hopping which may occur between poorly connected carbon rich regions. Thus, if the free carbon content of the silicon carbide fibers could be homogeneously reduced and/or the connecting paths between the carbon rich regions made more resistive on a microscopic scale, the conductivity and dielectric constant of the fibers would be further reduced.
One simple method of reducing the carbon content would be to oxidize the free carbon to carbon monoxide. However, heating silicon carbide fibers in air at temperatures of 1,000.degree. C. and above leads to the formation of an oxide coating. Such an oxide coating limits the oxidation of the interior of the fiber and, thus, limits the oxidation of free carbon to reduce the conductivity and dielectric constant of the fibers. The oxide coating also causes significant reductions in the strength of the fiber.
On the other hand, copending, commonly assigned U.S. Ser. No. 895,420, filed August 11, 1986, has found that treating SiC fibers at temperatures above 1,000.degree. C. in the presence of trace concentrations of oxygen while leading to the formation of an oxide coating on silicon carbide fibers, nevertheless, does not harm he mechanical properties of the fibers. Such treated fibers actually retain the weight and tensile properties of the original fibers even when aged in a severe environment of high temperature.
U.S. Pat. No. 4,283,376 discloses producing a silicon carbide fiber which is obtained from the pyrolysis of polycarbosilanes which contain borosiloxane groups. After the fibers have been pyrolyzed to the ceramic, impurities such as graphite, free carbon or silica are removed by heating the pyrolyzed fibers at a temperature of preferably 800.degree. to 1,600.degree. C. in an atmosphere of at least one gas selected from oxygen, air, ozone, hydrogen, steam and carbon monoxide. The patent states that when the decarbonization treatment is performed at a temperature of not more than 800.degree. C., free carbon cannot be fully removed and that when the heating temperature exceeds 1,600.degree. C., the reaction of SiC and the atmospheric gas takes place vigorously.
U.S. Pat. No. 4,399,232 is like the preceding patent and is concerned with forming inorganic fibers obtained from pyrolyzing organosilicon fibers which contain polycarbosilane blocks and titanoxane units. The patent states that free carbon can be removed from pyrolyzed fibers by heating the fibers in an atmosphere of at least one gas selected from the group consisting of oxygen gas, air, ozone, hydrogen gas, steam and carbon monoxide gas, preferably at a temperature of 800.degree. to 1,600.degree. C. Again, this patent states upon heating below 800.degree. C., the free carbon cannot be fully removed.
Accordingly, it would be advantageous to provide silicon carbide fibers of increased resistivity and consequently increase the uses of such fibers. Obviously, it would be advantageous to increase the resistivity of silicon carbide fibers without adversely affecting the mechanical properties of such fibers.
The primary object of the present invention is thus to increase the resistivity and lower the dielectric constant of ceramic fibers formed by the pyrolysis of organosilicon preceramic polymers and to accomplish same without adversely affecting the desirable properties of the fibers.
Another object of the invention is to reduce the free carbon content of silicon carbide fibers formed from organosilicon preceramic polymers.
Still another object of the present invention is to provide a process for oxidizing the free carbon which is in the interior of a silicon carbide fiber formed from the pyrolysis of organosilicon preceramic polymers.
Still yet another object is to produce a highly resistive silicon carbide fiber which can be used in resin composites or otherwise formed into articles which will find use in electronic applications.
These and other objects, aspects and advantages of the invention will be readily apparent to those of ordinary skill in this art upon consideration of the following description of the invention and the appended claims.